Five takeaways from the first day of the Parkland school shooting trial.

The first day of the trial over whether the gunman who killed 17 people in Parkland, Fla., in 2018 should be sentenced to death or life in prison included disturbing videos taken inside classrooms where several students were shot.

Several jurors covered their mouths with their hands on Monday as the videos were played, and they watched students cowering in fear while the gunman stalked the hallways of Marjory Stoneman Douglas High School with a semiautomatic rifle.

The judge barred the video images from being seen by the public during the trial, but the audio from the video clips permeated the courtroom. Students could be heard yelling for help, whispering to one another and admonishing others to stay quiet as they hid and waited for help.

Monday was the harrowing start of what is expected to be a monthslong sentencing trial, culminating with jurors deliberating over whether the gunman, Nikolas Cruz, 23, should be executed or imprisoned for the rest of his life. He pleaded guilty in October to 17 counts of first-degree murder, as well as 17 counts of attempted murder relating to people he wounded.

Here are five takeaways from the first day of proceedings:

• Gut-wrenching evidence. Relatives of the victims sobbed in the courtroom as videos from inside one classroom where four people were shot were played for the jurors. The audience heard but did not see the videos, which were taken by Danielle Gilbert, who was a junior in the school at the time. A boy in her classroom could be heard shouting, “Someone help me!” One woman listening in the audience was so overwhelmed that she left the courtroom.

• “Systematic” murder. In his opening statement, the lead prosecutor, Michael J. Satz, described what he called the “systematic” murder of 14 students and three school staff members by the gunman. “I’m going to speak to you about the unspeakable,” Mr. Satz said. He noted that immediately after the shooting, the gunman left the school, went to Subway and ordered a frozen drink before being apprehended. Defense lawyers chose to put off giving their opening statement until after prosecutors have called all of their witnesses.

• Valentine’s Day cards. Brittany Sinitch, a teacher at the high school who was also an alumna, testified that her students were working on a Valentine’s Day-themed project — writing cards to one another as characters in “Romeo and Juliet” — on Feb. 14, 2018, when she heard gunshots. The noise of the gunfire was so loud, Ms. Sinitch said, that when she called 911 to report the shooting, a dispatcher could not hear what she was saying.

• Harrowing memories. Another junior at the school during the shooting, Dylan H. Kraemer, described looking around his classroom and seeing two classmates who had been killed after the gunman peered through the classroom door’s window and fired into the room. So many students were hurt in the classroom, Mr. Kraemer said, that he could not recall all of their names. As prosecutors played another video from inside a classroom during his testimony, the parents of Helena Ramsay, a student who was killed in that classroom, shouted for it to be turned off.

• Mistrial denied. Defense lawyers asked the judge to declare a mistrial, arguing that playing the video for the jury during Mr. Kraemer’s testimony, with gunshots audible in the court at full volume, was inflammatory and unnecessary, and that the reaction of the parents who shouted for it to be turned off could be prejudicial. Judge Elizabeth A. Scherer denied the request. Defense lawyers often make several mistrial motions during a trial, in part to bolster possible appeals if the case ends in an unfavorable verdict.

Until now, no American who had killed as many people as Mr. Cruz did in a mass shooting has faced trial. Under Florida law, a death sentence would require a unanimous recommendation by the jury; otherwise, the mandatory sentence is life in prison without possibility of parole.